Magnetic recording heads have utility in magnetic disc drive storage systems. Most magnetic recording heads used in such systems today are “longitudinal” magnetic recording heads. Longitudinal magnetic recording in its conventional form has been projected to suffer from superparamagnetic instabilities at high bit densities.
Superparamagnetic instabilities become an issue as the grain volume is reduced in order to control media noise for high areal density recording. The superparamagnetic effect is most evident when the grain volume V is sufficiently small that the inequality KuV/kBT>70 can no longer be maintained. Ku is the material's magnetic crystalline anisotropy energy density, kB is Boltzmann's constant, and T is absolute temperature. When this inequality is not satisfied, thermal energy can demagnetize the stored data bits. Therefore, as the grain size is decreased in order to increase the areal density, a threshold is reached for a given material Ku and temperature T such that stable data storage is no longer feasible.
The thermal stability can be improved by employing a recording medium formed of a material with a very high Ku. However, with the available materials the recording heads are not able to provide a sufficient or high enough magnetic writing field to write on such a medium. Accordingly, it has been proposed to overcome the recording head field limitations by employing thermal energy to heat a local area on the recording medium before or at about the time of applying the magnetic write field to the medium. By heating the medium, the Ku or the coercivity is reduced such that the magnetic write field is sufficient to write to the medium. Once the medium cools to ambient temperature, the medium has a sufficiently high value of coercivity to assure thermal stability of the recorded information. Heat assisted magnetic recording allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure a sufficient thermal stability. Heat assisted magnetic recording can be applied to any type of magnetic storage media, including tilted media, longitudinal media, perpendicular media and patterned media.
It is believed that reducing or changing the bit cell aspect ratio will extend the bit density limit. However, different approaches will likely be necessary to overcome the limitations of longitudinal magnetic recording.
An alternative to longitudinal recording that overcomes at least some of the problems associated with the superparamagnetic effect is “perpendicular” magnetic recording. Perpendicular magnetic recording is believed to have the capability of extending recording densities well beyond the limits of longitudinal magnetic recording. Perpendicular magnetic recording heads for use with a perpendicular magnetic storage medium may include a pair of magnetically coupled poles, including a main write pole having a relatively small bottom surface area and a flux return pole having a larger bottom surface area. A coil having a plurality of turns is located adjacent to the main write pole for inducing a magnetic field between the pole and a soft underlayer of the storage media. The soft underlayer is located below the hard magnetic recording layer of the storage media and enhances the amplitude of the field produced by the main pole. This, in turn, allows the use of storage media with higher coercive force, consequently, more stable bits can be stored in the media. In the recording process, an electrical current in the coil energizes the main pole, which produces a magnetic field. The image of this field is produced in the soft underlayer to enhance the field strength produced in the magnetic media. The flux density that diverges from the tip into the soft underlayer returns through the return flux pole. The return pole is located sufficiently far apart from the main write pole such that the interaction of the magnetic flux from the main write pole with the magnetic material of the return pole is negligible. The magnetic flux from the main write pole is directed vertically into the hard layer and the soft underlayer of the storage media.
When applying a heat or light source to the medium, it is desirable to confine the heat or light to the track where writing is taking place and to generate the write field in close proximity to where the medium is heated to accomplish high areal density recording. In addition, for heat assisted magnetic recording (HAMR) one of the technological hurdles to overcome is to provide an efficient technique for delivering large amounts of light power to the recording medium confined to spots of, for example, 50 nm or less. A variety of transducer designs have been proposed and some have been experimentally tested. Among those are metal-coated glass fibers and hollow pyramidal structures with metal walls. For all these approaches, confinement of the light depends on an aperture which is fabricated into the end of structure and which gives this kind of transducer the name “aperture probes.” Generally these devices suffer from very low light transmission rendering the devices useless for HAMR recording. For example, tapered and metallized optical fibers have demonstrated light confinement down to approximately 50 nm with a throughput efficiency of 10−6. Pyramidal probes made from anisotropic etching of Si wafers have been designed with throughput efficiencies of 10−4 for similar spot sizes. Although this is the state of the art, it is still about two orders of magnitude too small for HAMR.
Improvements in throughput efficiency have been achieved for these transducers by changing the taper angles, filling the hollow structures with high index materials, and by trying to launch surface plasmons (SP) on integrated edges and corners of these tip-like structures. Although doing so does increase the throughput to some extent, the most promising SP approach is still very inefficient due to a lack of an efficient SP launching technique. In addition, all aperture probes suffer from a lower limit on spot size which is twice the skin depth of the metal film used to form the aperture. Even for aluminum, the metal with the smallest skin depth for visible light, this corresponds to a spot size of ˜20 nm.
Accordingly, there is a need for devices that can provide a reduced spot size and increased throughput efficiencies. Such devices can be used in heat assisted magnetic recording heads.